Lassa virus is a segmented negative-sense RNA virus that belongs to the family Arenaviridae. Arenaviruses are further sub-divided into the Old World and New World virus complexes based on serological cross-reactivity, phylogenetic relations, and geographical distribution, (Wulff, 1978; Bowen, 1997). The New World arenavirus complex comprises viruses that circulate in North America (i.e., Whitewater Arroyo (WWAV), Tamiami (TAMV), and Bear Canyon (BCNV) viruses) and South America (i.e., Tacaribe (TACV), Junin (JUNV), Machupo (MACV), Guanarito (GTOV), and Sabia (SABV) viruses). The Old World complex includes arenaviruses that circulate in Africa, Europe, and Asia (i.e., lymphocytic choreomeningitis (LCMV) and Lassa (LASV) viruses). The range of reservoir rodent species restricts the geographic occurrence of arenaviruses, with the exception of LCMV that is distributed worldwide due to its association with Mus domesticus and M. musculus, which have migrated globally (Salazar-Bravo, 2002). The reservoir hosts of LASV are rodents of the genus Mastomys that are enzootic in sub-Saharan Africa (Salazar-Bravo, 2002). At least seven arenaviruses are known to cause severe hemorrhagic fever in humans, among which are LASV, JUNV, MACV, GTOV, and SABV that are endemic in West Africa, Argentina, Bolivia, Venezuela, and Brazil, respectively, and recently discovered Lujo (LUJV) and Chapare (CHAPV) viruses that originated in Zambia and Bolivia, respectively (Breise, 2009; Delgado, 2008).
Lassa virus (LASV) is endemic to West Africa with an estimated 300,000-500,000 people infected annually (McCormick, 1987). Transmission occurs through contact with infected rodents (Mastomys natalensis) or virus-contaminated rodent excreta, and person-to-person transmission, especially in hospital settings, has been documented (McCormick, 1987). Disease caused by LASV ranges from subclinical infection to mild to severe hemorrhagic fever that is associated with multi-organ failure. Mortality rates associated with LASV infection vary and range from approximately 2% to 15% for hospitalized cases and can exceed 50% in certain outbreak scenarios (McCormick, 1987; Fisher-Hoch, 1995). Despite the high incidence and associated morbidity and mortality, there is no approved therapy to treat LASV infection in humans. Supportive care and early administration of ribavirin are current standard of care.
LASV initially infects monocytes, macrophages, and dendritic cells and spreads systemically to produce a primary viremia that leads to infection of internal organs. Virus replication leads to a rise in inflammatory cytokine levels and development of coagulopathies resulting in vascular leakage, hypovolemic shock and multi-organ failure (Hensley, 2011).
Replication of arenaviruses is catalyzed by the L polymerase protein that utilizes viral RNA templates that consist of genomic RNA encapsidated by the viral nucleocapsid protein NP and comprises viral ribonucloprotein (RNP) (Buchmeier, 2007). Replication is initiated upon viral entry into the host cell where the L polymerase, associated with the viral RNP, initiates transcription from the genome promoter located at the 3′-end of each genomic RNA segment, L and S. The primary transcription event results in the synthesis of NP and L polymerase mRNA encoded in antigenomic orientationfrom the S and L segments, respectively. Transcription terminates at the distal side of the stem-loop (SL) structure within the intergenomic region (IGR). Arenaviruses utilize a cap snatching strategy to acquire the cap structures of cellular mRNAs to facilitate translation. Cap snatching is mediated by the endonuclease activity of the L polymerase that is co-factored by the cap binding activity of NP to produce capped non-polyadenylated mRNAs. Subsequently, the L polymerase adopts a replicase mode and moves across the IGR to generate a full-length complementary antigenomic RNA (agRNA). This agRNA serves as a template for the synthesis of GPC and Z mRNAs encoded in genomic orientationfrom the S and L segments, respectively, and for the synthesis of full-length genomic RNA (gRNA) (Buchmeier, 2007; Franze-Fernandez, 1987; Meyer, 1993; Qi, 2010; Lelke, 2010; Morin, 2010).
Human coronaviruses, first identified in the mid-1960s, are common viruses that infect most people at some time in their life, generally causing mild to moderate upper respiratory and gastrointestinal tract illnesses. The novel coronavirus referred to as “Middle East Respiratory Syndrome Coronavirus” (MERS-CoV or MERS) was first reported in Saudi Arabia in 2012 and has spread to several other countries. SARS-CoV, the coronavirus responsible for Severe Acute Respiratory Syndrome (SARS) was first recognized in China in 2002 and led to a worldwide outbreak in 2002 and 2003.